Abstract 2816: Evaluation of Cdk5 inhibitors in neural crest tumors

Abstract

Abstract Cyclin-dependent kinase 5 (Cdk5) is a ubiquitously expressed proline-directed serine threonine kinase active mainly in post-mitotic neurons due to abundant expression of its obligate activating partners p35 and/or p39. Cdk5 has been viewed narrowly as a neuron-specific kinase and as an essential regulator of neuronal function and migration. Recently, Cdk5 activity has been implicated in mediating tumor invasion, progression, and metastasis. We have found invariably high expression and activity of Cdk5/p35 in a series of pediatric tumor cell lines including neuroblastoma. To evaluate the potential role of Cdk5 inhibitors in the treatment of neural crest tumors, we evaluated the in vitro kinase activity and growth inhibition of 4 Cdk5 inhibitors (AT7519, dinaciclib, flavopiridol, and seliciclib). We compared the in vitro activity against Cdk2/cyclin E, Cdk5/p25, Cdk5/p35, Cdk7/cyclin H, and Cdk9/cyclin K. Using a DNA-based proliferation assay, we assessed the in vitro growth inhibition in a panel of 10 neural crest-derived tumors (melanoma, neuroblastoma, Ewing sarcoma). All Cdk5 inhibitors demonstrated enzyme and growth inhibition at clinically achievable concentrations. Dinaciclib was the most potent inhibitor of Cdk5/p35 with an enzyme IC50 of 0.0002 μM, compared to 0.003 μM for AT7519, 0.021 μM for flavopiridol, and 0.097 μM for seliciclib. Dinaciclib was also the most potent inhibitor of Cdk2. However, flavopiridol and AT7519 were the most potent inhibitors of Cdk7 and Cdk9, respectively. Across all cell lines, dinaciclib was the most potent inhibitor of growth. The median (range) growth inhibition IC50 for dinaciclib was 0.006 μM (0.004 - 0.14), compared to 0.079 μM (0.045-0.16) for flavopiridol, 0.52 μM (0.14 - >15) for AT7519, and 13.2 μM (7.8 - 22.1) for seliciclib. Interestingly, within the Ewing sarcoma cell lines, there was disparate sensitivity to the 2 most potent Cdk5 inhibitors, dinaciclib and AT7519. Based on the activity of these Cdk inhibitors in neural-crest tumors, we expanded our evaluation of AT7519 in 6 pediatric tumor cell lines (medulloblastoma, rhabdomyosarcoma, osteosarcoma) and found similar in vitro potency with a median (range) IC50 of 0.43 μM (0.20 - 1.26). AT7519, dinaciclib, flavopiridol, and seliciclib demonstrated in vitro growth inhibition in a panel of neural crest and pediatric solid tumor cell lines at clinically achievable concentrations. Further in vivo evaluations and in vitro combination studies including this novel class of agents in models of neural crest and select pediatric cancers are warranted. Citation Format: Robin E. Norris, John J. Pink, Tej Pareek, John L. Letterio. Evaluation of Cdk5 inhibitors in neural crest tumors. [abstract]. In: Proceedings of the 107th Annual Meeting of the American Association for Cancer Research; 2016 Apr 16-20; New Orleans, LA. Philadelphia (PA): AACR; Cancer Res 2016;76(14 Suppl):Abstract nr 2816.